1. Field of the Invention
The present invention relates generally to neutron monitoring systems, and more particularly, to a neutron monitoring system that measures a neutron flux using neutron detectors placed inside a nuclear reactor or under the reactor vessel.
2. Description of the Related Art
Generally, neutron detectors are placed inside a nuclear reactor to monitor nuclear power of the reactor. For example, a boiling-water reactor includes an in-core instrumentation tube inserted in a gap of the fuel assemblies installed in a grid-like form inside the reactor. Inside the in-core instrumentation tube placed are neutron detectors such as fission chambers for monitoring of nuclear power. Upon entry of a neutron, the neutron detectors generate an electric signal and send a measured neutron flux signal to a power monitoring device installed outside the reactor pressure vessel, through independent cables connected to the detectors. The neutron detectors here detect the neutrons generated from the core fuel immersed in coolant water.
A water level gauge for measuring the coolant water level in the reactor is provided to confirm whether appropriate cooling is performed on the core fuel. An instrument with a differential pressure gauge is commonly used as the water level gauge. Further, a method of detecting the reactor coolant water level directly inside the reactor has come to be considered to ensure diversity of detection methods.
For example, Japanese Patent Application No. JP-1984-112290-A discloses a reactor core monitoring device that senses a coolant water level with a plurality of sheathed thermocouples placed in an in-core instrumentation tube of a boiling-water reactor. The sheathed thermocouples are placed at different heights from a lower section of the reactor, and differential output between upper and lower sheathed thermocouples are detected, thereby sensing the coolant water level.